1. Field of the Invention
This invention relates to an apparatus for eliminating a smear charge generated with a solid image pickup element of frame transfer type.
2. Description of the Related Art
A CCD solid image pickup element for use in a television camera, for example, receives light by a plurality of photoelectric converters arranged in a matrix, stores picture charges (information charges), and transfers the picture charges vertically and horizontally, thus outputting successive picture signals screen by screen.
FIGS. 7(a) and 7(b) of the accompanying drawings are diagrams each showing a CCD solid image pickup element of frame transfer type. The frame transfer type CCD comprises a picture area 1 in which a plurality of photoelectric converters are arranged in a matrix, a store area 2 in which a plurality of vertical registers are arranged in association to the columns of the picture area, and a horizontal register 3. The picture charges generated in the picture area 1 by the photoelectric conversion are temporarily transferred to the store area 2 screen by screen, and are then outputted from the store area 2 horizontal line by horizontal line via the horizontal register 3.
In such solid image pickup element, as shown in FIG. 7(a), if a strong spotlight 4 is irradiated on a portion of the picture area 1, a smear charge due to the spotlight 4 is superposed over the picture charges as the picture charges are transferred from the picture area 1 to the store area 2. Therefore, as shown in FIG. 7(b), permeation of light and thus a smear is produced in the area 5 which has the picture charges having passed the area irradiated by the spotlight 4. Consequently, as shown in FIG. 8, a smear 6 with respect to the spotlight 4 would be developed vertically in a reproduced picture.
In the solid image pickup element of frame transfer type in which the picture charges are transferred in the picture area normally irradiated by light, the above-discussed smear would be produced. To this end, a circuit for minimizing the production of such smear is disclosed such as in Japanese Patent Publication No. 35067/1981.
FIG. 9 is a block diagram showing a circuit for minimizing smear charges. In the CCD solid image pickup element 10 of FIG. 9, from one to several lowermost horizontal lines 1a of the picture area 1, in addition to the store area 2 and the horizontal register 3, are shielded off light, and the remaining portion 1b of the picture area 1 receives light. A multiphase drive clock is supplied to the solid image pickup element 10, and the picture charges obtained by the picture area 1 are outputted horizontal line by horizontal line from the store area 2 via the horizontal register 3.
A line memory 11 is connected to the output of the solid image pickup element 10 via a switch 12, and only signals which correspond to the charges stored in the horizontal lines 1a of the picture area 1 as shaded off light are stored by shifting the switch 12. In the shaded horizontal lines 1a, since light is not irradiated during taking a picture, only smear charges are stored. Thus picture signals X(t) obtained from the solid image pickup element 10 comprise a first kind of signals each including a picture component and a smear component, and a second kind of signals each including only a smear component, as shown in FIG. 10. At the timing of outputting the signals including only smear components, the switch 12 is shifted so as to fetch the picture signals X(t) in a line memory 11 for one horizontal line to store the smear charge signals in the line memory 11. From the line memory 11, as shown in FIG. 10, the smear charge signals S(t) are read in synchronism with the picture signals.
The output of the solid image pickup member 10 is connected to the positive input side of a subtracter 13, and the output of the line memory 11 is connected to the negative input side of the subtracter 13. When the picture signals X(t) each including a picture component and a smear component are inputted from the solid image pickup element 10, and when the smear charge signals S(t) including only smear components are inputted from the line memory 11, the smear charge signals S(t) is subtracted from the picture signals X(t). Thus the picture signals Y(t) which do not include any smear component, as shown in FIG. 8.
In this smear eliminating method, because any smear component of the picture signal is detected and subtracted screen by screen, precise detection of the smear components is necessary. Instead, the lowermost portion of the picture area 1 of the solid image pickup element 10 is shaded off light, and the charges stored in this shaded area la are employed in detecting any smear component.
However, in the solid image pickup element of frame transfer type in which vertical picture elements can be inadequately separated, even when the lowermost portion of the picture area 1 is shaded off light, there is the possibility that some charges permeate from the unshaded area 1b into the shaded area 1a. Thus charges are stored in the shaded area 1a particularly near the border to the unshaded area 1b. Specifically, the general solid image pickup element of frame transfer has a channel stop area which separates the picture elements vertically and separates the picture elements by a potential fence formed by a transfer electrode, so that it cannot be guaranteed that the charges created at the deep of the substrate are stored exactly in predetermined picture elements. Since the quantity of charges stored in the shaded area is more than the quantity of the original smear, accurate detection of the smear components is difficult to achieve.
When the detection of the smear components is inaccurate, elimination of the smear is inadequate so that the picture signals are deteriorated to degraded the quality of a reproduction screen.
If the shaded area 1a is wide, it is possible to prevent permeation of the charges; but, since the effective area of the picture area 1 is small, the shaded area 1a cannot be so large.